BACKGROUND: Breast milk is the main source of postnatal human cytomegalovirus (HCMV) infection. The aim of this study was to assess the relationship between HCMV load in breast milk and viral transmission to very low birth weight (VLBW) infants. METHODS: Breast-fed VLBW infants who were born to HCMV-seropositive mothers and who were managed in a neonatal intensive care unit were enrolled in the study. Blood from mothers and infants was tested for HCMV antibodies after birth. Breast milk was collected for viral culture and HCMV load measurement. Urine from the babies was obtained for HCMV-DNA detection. Symptoms of HCMV infection were recorded and evaluated by neonatologists. RESULTS: Of the 23 evaluated mothers during a 1-year period, 19 were HCMV seropositive; 17 of the women had detectable HCMV-DNA in their breast milk whey. Of the 23 infants born to the 19 seropositive mothers, 8 infants of 8 mothers had HCMV-DNA detected in the urine, indicating that they were infected, even though the breast milk was always frozen prior to feeding. Three infected infants had symptoms. At 4 weeks after delivery, the median viral load in breast milk from mothers of the 8 infected infants was significantly higher than that from mothers of the 15 noninfected infants (P = 0.04). HCMV was detectable in breast milk for a significantly longer period in mothers of infected infants (7.5 vs. 2.6 weeks P = 0.03). CONCLUSIONS: High HCMV load and prolonged virus excretion in breast milk are maternal risk factors for viral transmission to VLBW infants.